Yes, thanks!

Does this fix it?

Changes have been made in Git (FRAMEWORK_5_2):

commit 390d98c1eb357a8c602627a01eb5add9d3e8e109
Author: Jan Schneider <jan@horde.org>
Date:   Tue Nov 25 17:43:52 2014 +0100

     [jan] Fix sending reminders (Bug #12867).

  whups/docs/CHANGES |    1 +
  whups/package.xml  |    4 ++--
  2 files changed, 3 insertions(+), 2 deletions(-)

http://github.com/horde/horde/commit/390d98c1eb357a8c602627a01eb5add9d3e8e109

Changes have been made in Git (FRAMEWORK_5_2):

commit 6b9b264b8d12e92bbe5fc6c38a708575a0392aa6
Author: Jan Schneider <jan@horde.org>
Date:   Tue Nov 25 17:39:38 2014 +0100

     Fix sending reminders (Bug #12867).

  whups/lib/Driver.php |    5 ++++-
  1 files changed, 4 insertions(+), 1 deletions(-)

http://github.com/horde/horde/commit/6b9b264b8d12e92bbe5fc6c38a708575a0392aa6

Changes have been made in Git (master):

commit 56a7df71748c069f9e8186aca860a4cfcf4160f4
Author: Jan Schneider <jan@horde.org>
Date:   Tue Nov 25 17:39:38 2014 +0100

     Fix sending reminders (Bug #12867).

  whups/lib/Driver.php |    5 ++++-
  1 files changed, 4 insertions(+), 1 deletions(-)

http://github.com/horde/horde/commit/56a7df71748c069f9e8186aca860a4cfcf4160f4

Commit 7b096cdc9b3257349c3508d2cc8ac1902be88beb is the culprit.
https://github.com/horde/horde/commit/7b096cdc9b3257349c3508d2cc8ac1902be88beb

@ Jan,

can you reproduce this issue with this informations?

we just investigated this a bit further: problem seems to originate in 
this if-statement (Driver.php:515)

             if ($details && $details['type'] == 'user') {
                 $user_prefs = $GLOBALS['injector']
                     ->getInstance('Horde_Core_Factory_Prefs')
                     ->create('whups', array('user' => $details['user']));
                 if ($from['type'] == 'user' &&
                     $details['user'] == $from['user'] &&
                     $user_prefs->getValue('email_others_only')) {
                     continue;
                 }
             }

if we set email_others_only to 0, reminders are being sent. if we 
leave it to (default) 1, reminders are skipped because 
$details['user'] == $from['user']. this seems to originate from 
Whoups.php:960:

             $whups_driver->mail(array('recipients' => array($user => 'owner'),
                                       'subject' => $subject,
                                       'view' => $view,
                                       'template' => $message_file,
                                       'from' => $user));

where the same user is set in the recipients list as in the from field.

             /* We may have no recipients due to users excluding themselves
              * from self notifies. */
             if (!$to) {
                 continue;
             }
             if ($details && $details['type'] == 'user') {
                 $user_prefs = $GLOBALS['injector']
                     ->getInstance('Horde_Core_Factory_Prefs')
                     ->create('whups', array('user' => $details['user']));
                 if ($from['type'] == 'user' &&
                     $details['user'] == $from['user'] &&
                     $user_prefs->getValue('email_others_only')) {
                       Horde::debug($from);
                        mail('samuel.wolf@wolf-maschinenbau.de','test--1', 
'nachrichttext');
                     continue;
                 }
             }
             mail('samuel.wolf@wolf-maschinenbau.de','test--2', 
'nachrichttext');
             if ($opts['ticket']) {
                 /* Add attachments. */


I receive "test--1", but never "test--2".


Horde Debug:

2014-10-13T19:28:18+02:00 DEBUG: Variable information:
array(4) {
   ["user"]=>
   string(11) "samuel.wolf"
   ["type"]=>
   string(4) "user"
   ["name"]=>
   string(11) "Samuel Wolf"
   ["email"]=>
   object(Horde_Mail_Rfc822_Address)#607 (4) {
     ["comment"]=>
     array(0) {
     }
     ["mailbox"]=>
     string(11) "samuel.wolf"
     ["_host":protected]=>
     string(20) "wolf-maschinenbau.de"
     ["_personal":protected]=>
     NULL
   }
}

../whups/config/prefs.local.php
$_prefs['email_others_only']['value'] = 1;

whups/lib/Driver.php

In which file?

Your install may, or may not be using PHP's mail() function - it 
depends on the transport you have configured.

Anyway, next you should look in Whups_Driver::mail() to see where 
things are failing. The mail is sent around line 614 .. $mail->send()

$view->tickets = $utickets;
$subject = _("Reminder: Your open tickets");

Horde::debug($subject);
mail('samuel.wolf@wolf-maschinenbau.de','test', 'nachrichttext');

$whups_driver->mail(array('recipients' => array($user => 'owner'),
                                       'subject' => $subject,
                                       'view' => $view,
                                       'template' => $message_file,
                                       'from' => $user));

PHP mail work, I receive the test mail.

Add debug info in Whups.php:
###########################################################################
             $view->tickets = $utickets;
             $subject = _("Reminder: Your open tickets");
             $whups_driver->mail(array('recipients' => array($user => 'owner'),
                                       'subject' => $subject,
                                       'view' => $view,
                                       'template' => $message_file,
                                       'from' => $user));

             Horde::debug($subject);
###########################################################################

horde_debug.txt show then:
###########################################################################
2014-10-11T18:51:45+02:00 DEBUG: Variable information:
string(32) "Erinnerung: Ihre offenen Tickets"

Backtrace:
1. Whups::sendReminders() /var/www/https/horde/whups/admin/index.php:1133
2. Horde::debug() /var/www/https/horde/whups/lib/Whups.php:974

2014-10-11T18:51:45+02:00 DEBUG: Variable information:
string(32) "Erinnerung: Ihre offenen Tickets"

Backtrace:
1. Whups::sendReminders() /var/www/https/horde/whups/admin/index.php:1133
2. Horde::debug() /var/www/https/horde/whups/lib/Whups.php:974

[...]
###########################################################################

I guess that is the output we like to see and the problem is somewhere else.
Is there a way to pass the mail array into horde_debug.txt?

Whats the next step?

Note: Send mail on ticket creation/edit works.

The sending is done in Whups::sendReminders(), in lib/Whups.php

Is there a point to start track down this issue?
whups 3.0.0 stable
horde 5.2.0 stable

Those are harmless PHP notices.

better to read...

Horde DEBUG log:
[...]
2013-12-05T23:17:45+01:00 DEBUG: HORDE [whups] PHP ERROR: Declaration 
of Horde_Form_Type_invalid::init() should be compatible with 
Horde_Form_Type::init() [pid 24373 on line 3839 of 
"/usr/share/php/Horde/Form/Type.php"]
2013-12-05T23:17:45+01:00 DEBUG: HORDE [whups] PHP ERROR: Declaration 
of Horde_Form_Type_country::init() should be compatible with 
Horde_Form_Type::init() [pid 24373 on line 908 of 
"/usr/share/php/Horde/Form/Type.php"]
2013-12-05T23:17:45+01:00 DEBUG: HORDE [whups] SQL  (0.0001s)
[...]
2013-12-05T23:17:45+01:00 DEBUG: HORDE [whups] PHP ERROR: Only 
variables should be assigned by reference [pid 24373 on line 47 of 
"/var/www/https/horde4/whups/lib/Form/SendReminder.php"]
[...]
2013-12-05T23:17:45+01:00 DEBUG: HORDE [whups] SQL  (0.0000s)
         SELECT ticket_id as id, ticket_owner as owner FROM whups_ticket_owners
           WHERE ticket_id = 723 [pid 24373 on line 319 of 
"/usr/share/php/Horde/Db/Adapter/Mysqli.php"]
2013-12-05T23:17:45+01:00 DEBUG: HORDE [whups] Reminders were sent. 
[pid 24373 on line 27 of 
"/usr/share/php/Horde/Core/Notification/Handler/Decorator/Hordelog.php"]
2013-12-05T23:17:45+01:00 DEBUG: HORDE [whups] SQL  (0.0004s)
[...]
2013-12-05T23:17:45+01:00 DEBUG: HORDE [whups] PHP ERROR: Only 
variables should be assigned by reference [pid 24373 on line 201 of 
"/usr/share/php/Horde/Form/Renderer.php"]
2013-12-05T23:17:45+01:00 DEBUG: HORDE [whups] PHP ERROR: Only 
variables should be assigned by reference [pid 24373 on line 232 of 
"/usr/share/php/Horde/Form/Renderer.php"]
2013-12-05T23:17:45+01:00 DEBUG: HORDE [whups] PHP ERROR: Only 
variables should be assigned by reference [pid 24373 on line 201 of 
"/usr/share/php/Horde/Form/Renderer.php"]
2013-12-05T23:17:45+01:00 DEBUG: HORDE [whups] PHP ERROR: Only 
variables should be assigned by reference [pid 24373 on line 232 of 
"/usr/share/php/Horde/Form/Renderer.php"]
2013-12-05T23:17:45+01:00 DEBUG: HORDE [whups] Max memory usage: 
20185088 bytes [pid 24373 on line 598 of 
"/usr/share/php/Horde/Registry.php"]
[...]

Yes ;-)
Will debug it on the weekend, maybe there some more information in the 
horde debug log.

I assume the assignee has an email address set in his preferences? 
Other than that, no idea.

That is not the problem:
* State Name [ Zugewiesen ]
* State Description [ Someone has accepted responsibility for the ticket. ]
* State Category [ Assigned ]

There's a difference between states and state categories. You can name 
your states any way you like, but make sure that your "Zugewiesen" 
state is of the state *category* "Assigned/Zugewiesen".

In my case "Zugewiesen", it work definitely in early versions RC1 or Beta.

You need states of the category "Assigned" and tickets need to be set 
to this state.

Is it a problem that we customize the stats?
If not, RC3 missing some git commits or I forget a config for send reminders?

Cannot reproduces, reminders are sent to assignees instantly here.

If I send reminders for a selected queue the notification comes up 
with "Reminders were sent.", but not one email were send.
No errors in the php error log.